Epidemiology: confounding: practice question

A PSM question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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PSM · Epidemiology: confounding

Sample item

A study finds that regular tea drinkers have a higher frequency of oesophageal cancer than non-drinkers. Tea drinkers in this population are also more likely to chew tobacco, and tobacco independently causes oesophageal cancer. Tobacco chewing is best described as:

AA source of recall biasBAn effect modifierCA confounderDAn intermediate variable on the causal pathway

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. A confounder

A confounder is associated with the exposure, is an independent risk factor for the outcome, and is not an intermediate step between them. Tobacco meets all three conditions here and can create or exaggerate an apparent association between tea and cancer. It can be handled by restriction, matching or randomisation in design, and by stratification or multivariable analysis in the analysis.

Option by option

A. A source of recall bias
Recall bias arises from differential reporting of exposure and is a type of information bias, not a third variable.
B. An effect modifier
An effect modifier changes the strength of the association across its strata; it is not a source of spurious association.
C. A confounder (correct)
Correct. Tobacco is linked to both tea drinking and the cancer and is not caused by tea drinking.
D. An intermediate variable on the causal pathway
An intermediate variable lies between exposure and outcome, but tea drinking does not cause tobacco chewing.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this PSM question

What is the answer to this PSM question?

The best answer is C, A confounder. A confounder is associated with the exposure, is an independent risk factor for the outcome, and is not an intermediate step between them.

Why is "A source of recall bias" not the answer?

Recall bias arises from differential reporting of exposure and is a type of information bias, not a third variable.

Why is "An effect modifier" not the answer?

An effect modifier changes the strength of the association across its strata; it is not a source of spurious association.

Why is "An intermediate variable on the causal pathway" not the answer?

An intermediate variable lies between exposure and outcome, but tea drinking does not cause tobacco chewing.

What concept does this question test?

It tests Epidemiology: confounding in PSM. Tobacco meets all three conditions here and can create or exaggerate an apparent association between tea and cancer. It can be handled by restriction, matching or randomisation in design, and by stratification or multivariable analysis in the analysis.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Epidemiology: confounding?

Practise biostatistics with small calculations until sensitivity, predictive values and risk measures are automatic. Then use the flashcards and the other PSM questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.